As it is known, there exists portable work equipment, such as chainsaws and the like, which comprises moving cutting parts and which exposes users to a high risk of injury. As a matter of fact, while it is relatively easy to arrange shields and/or accident-prevention protection elements on fixed equipment and machine tools, so as to avoid any accidental contact with moving parts, this is not feasible on portable equipment.
Safety arrangements for chainsaws and similar tools have already been suggested, specifically dedicated to solving the safety problems connected with the use of such devices. In particular, two different approaches have been put forward, one based on a mechanical passive system and another one based on a remote-detection system. In both cases it is intended to promptly detect a condition of excessive closeness between an operator's limbs and the moving parts (such as the cutting chain of a chainsaw) to stop in time the operation of the tool and to hence limit the damage resulting from a possible direct contact.
WO2007060698, in the name of the same Applicant, shows an example of safety system for such equipment, wherein the approach of the remote detection of the relative position between cutting tool and operator is used.
In this publication it is suggested to use a radio-frequency transmission system for detecting the relative position of the operator with respect to the blade of a chainsaw. The receiving-transmitting system in radio frequency operates similarly to a condenser system, obtaining a signal proportional to the distance between two electrodes, one inserted in an item of safety clothing of the operator and the other one in the chainsaw blade portion.
In other prior art disclosures, such as U.S. Pat. No. 5,942,975, the use of capacitive systems for detecting a capacitive difference between two electrodes for determining the relative distance thereof is disclosed.
However, these well-known systems generally require the use of suitable garments wherein a circuit and active electronic equipment are inserted which makes up one of the two capacitive plates of the system. Moreover, they do not have an optimal operation, because the emission of the safety signal is affected by environmental conditions (for example, air humidity, operator's mass and clothing, and others).
WO9712174 discloses another example of safety system applied—among other things—to a cutting tool. In this case, the distance between a receiver and a transmitter (possibly integral with an operator's body) is detected through the reading of the absolute value of a transmitted signal. This implies problems of sensitivity and adaptation to the specific conditions, so much so that a grounding arrangement is suggested in order not to trigger an undesired deactivation of the tool. Such system has proved cumbersome and poorly applicable in the field of portable cutting tools which are suited to operate in highly changeable conditions.
US2010180740 discloses a safety system provided with a calibration unit. The calibration unit is provided for changing the warning threshold of significant quantities detectable by a sensor unit, which sensor is hence intended for activating an emergency signal still upon the exceeding of an absolute (calibrated) threshold value of the detected quantities. This implies simply adapting the reference threshold to the boundary conditions, with loss of system sensitivity. Moreover, the calibration unit has a complex operation, resorting to a detection device separate from the sensor unit for detecting values of some reference quantities to be entered in a feedback loop and compared against default values prestored in a suitable memory. The use of absolute reference thresholds implies that the operator must cooperate, lying at a short distance from the tool for the calibration operation to be completed.